how to put a drain valve on air compressor

by RF
(calgary, canada)

Hi Bill, i've got a compressor that doesn't have a drain, can you please give me some advice how i could add a drain valve on my compressor? Thanks

RF, it is odd to have a compressor tank without a drain, as every time a compressor runs, it's pumping water into the tank too.

Assuming it is a bona fide air tank that you have, then the route I use would be, with the tank empty of air completely of course...

  • acquire the tank drain

  • drill a 1/4-3/8" hole into the tank at the lowest spot on the tank

  • figure out what the male thread size is on the drain

  • weld a nut of the appropriate size to the exterior of the tank, over the hole (the weld must be pressure resistant up to the tank burst pressure - not a job for an amateur

  • thread the drain into the nut thread using an appropriate sealant

Or, get a compressor tank that is already so equipped (you can buy used compressors pretty cheaply) and relocated your pump to the new tank.

Good luck,


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Does a compressor tank need more than one port?

by Scott
(Ontario, Canada)

I have a compressor with a 20 gallon tank that has developed a pin hole leak and needs to be replaced. I have a replacement tank available which has a more than adequate rating (removed from a Natural Gas Vehicle conversion so rated greater than 1000 psi).

This tank however has only one port instead of the two ports on my existing tank. One port is used for compressed air in from the 'head' and the second is for compressed air out to the regulator, filter, etc. (Actually there are three if you include the drain port).

My question is, if I use this tank by teeing it into a line connected directly between the compressor head and the regulator, will it operate OK or will there be losses that will decrease the overall cfm rating? The tank would be installed 'inverted' so the port would be facing down and a 'drop pipe' installed directly below the port with an automatic drain.

All new units I see at Home Depot and the auto supply stores all have separate connections to the tank for 'inlet' and 'outlet' air so I assume there must be a good reason.

Any recommendations?

Bill answers...

Hello Scott:

Let's look at what the ports do.

One port, as you point out, is for the drain on the bottom of the tank, and this port is very important. You need to drain the tank regularly to help eliminate rust, and to help stop compressor generated water from flowing down your air line. Since the drain port is on the bottom of the tank, and outlets are on the top, free water generated would tend to stay in the bottom of the tank until it was drained.

Port two is the inlet port from the compressor head into the tank. This line is checked, meaning that the air only flows one way, from the compressor itself and into the tank. The air cannot flow back up to the compressor head, and once the compressor unloads, no air pressure can build up on the piston(s) head from the tank to prevent start up.

Port three is the discharge port, leading to (preferably) a filter, then a regulator so that you can modulate the downstream air pressure to your needs.

Now, let's combine the ports.

When the compressor pressure switch calls the compressor to start, air would flow down the line from the compressor, past the check valve, and into the tank. At the same time compressed air would flow into the air line leading to your FR & L, and to your application.

If your air-consuming application was running, the compressor would never generate enough air pressure to reach kick out, might not even reach MOP for the application, and it would run until it overheated and kicked out on thermal overload.

Having said that, if your air line has a checked coupler on the far end (air can't escape unless a connector is inserted into it), and there was no air consuming appliance connected and running, then the air line too would be part of the air reservoir / tank.

This would lead to a fair amount of water vapour and water in that air line since as it condensed, water would have no where to go but down the airline to your tools.

The reservoir / air tank of the compressor allows some dwell time for the compressed air to cool and drop out water, and now this warm compressed air would cool in the air line where water vapour would condense.

Here's more information about compressor generated water.

So, from a strictly plumbing standpoint what you present seems reasonable, if unpractical, as long as you are OK with lots of water running into your air lines. Make sure you've got a check valve in the line between the compressor and the Tee.

Good luck with this experiment. If you go ahead with it, how about an update after you get it working? And maybe send a picture?



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air compressor blows air out the bottom of tank screw

by karen givani
(reno, nevada usa)

my air compressor turns on, then blows air out the bottom of the main tank, the screw seems to be gone, not on the bottom,
Karen, if you are saying that as the air compressor is running air is blowing out of the tank drain valve, then the valve either needs to be shut, or replaced with one that works.

I can't see where on the compressor that you are having the problem. The tank drain is normally at the lowest point on the compressor air tank, and it is supposed to be opened and the tank drained daily.

If someone has opened that drain, turning the nut or the knurled ring, or the wing nut, should shut it.


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Tank water drain valve

by bill wuttke
(wisbech cambs england)

Hi, when i open the water drain on my 150l tank it does not always rush with air/water can i remove the whole valve without it leaking bill.
Bill says...

Howdy, Bill.

You say that when you open the tank drain valve that water doesn't always rush out. Is there water to rush out?

Certainly, if the tank has air in it, when you open the drain valve, compressed air should blow out the drain valve, and carry any water there along with it.

Can you remove the drain valve without it leaking? Does this mean that you might damage it when you remove it?

The tank drain valve is normally threaded into a tank fitting, and powerful sealant is often used to help ensure that the threads don't leak. So, it is possible that you will damage it during removal.

Replacing the tank drain valve is pretty easy if you have some fundamental plumbing experience. One thing that might be useful is to thread and elbow into where the tank drain used to be, add some tube so that it extends to the side of the tank, and add a shut off valve there. Then draining your tank becomes very easy.

Good luck.



Comments for Tank water drain valve

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Nov 13, 2017
Use tank drain as air port
by: Bill

Sure you can, but you shouldn't. If you do, all the water that collects as the compressor runs will flow right down the air line you've connected, and right to your air tools.

I'm curious as to why you'd want to do that?

Nov 13, 2017
Connecting at Drain Port
by: Noel

Hi Bill,

Can you use the drain port of the Air Compressor as a regular air out port?


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